Summary:
The spirit unit summoned by Elder Titan's Astral Spirit ability is not selected by "Auto-select Summoned Units", "Select All Controlled Units", and "Select All Other Units".

This was tested in Hero Demo Mode, in a bot match, in a private lobby, and in an online unranked match, and the same results occurred each time. The same results also occurred when stealing the spell as Rubick (only tested in Hero Demo mode).

This behavior is not consistent with the behavior of other abilities. Similar abilities/items that summon units function as they should, with all 3 previously-mentioned selection methods working properly, which indicates that this problem is exclusive to Elder Titan's Astral Spirit ability. I tested the following in Hero Demo mode, which all worked (the units are selected when the aforementioned methods are used): Necronomicon, Manta Style, Arc Warden's Tempest Double, Naga Siren's Mirror Image, Phantom Lancer's Spirit Lance/Doppelganger/Juxtapose, Lone Druid's Spirit Bear, Meepo's Divided We Stand, Chaos Knight's Phantasm, Visage's Summon Familiars, Invoker's Forge Spirits, Warlock's Chaotic Offering, Terrorblade's Conjure Image, Nature's Prophet's Nature's Call/Wrath of Nature with Aghs, Enigma's Demonic Conversion, Venomancer's Plague Wards, and Morphling's Replicate.

The only significant exception I could find that functions similarly to Astral Spirit was Beastmaster's Call of the Wild: Hawk ability. The hawk is neither selected by "Select All Controlled Units" nor by "Select All Other Units". However, it is selected if summoned while "Auto-select Summoned Units" is turned on. The boar, on the other hand, can be selected by all 3 methods. I can see this as being intended, because the hawk is generally left in one location to give vision, while the boar is much more actively controlled, but Elder Titan's Astral Spirit is not really similar to the hawk in this way.

I considered whether or not this might be intended, but since it's necessary to move the spirit through enemies to damage them/get the move speed/damage buff, as well as moving it to set up Echo Stomps, I think it would be useful to at least be able to use "Select All Other Units", so that it's easier to move the spirit separately from Elder Titan.

The spirit can be selected by left-clicking it, by box-selecting it (left-click and drag), or by adding it to a control group and then selecting that control group, but the 3 other methods mentioned earlier do not work. Note that adding the spirit to a control group will always cause it to be selected when selecting that control group, even on subsequent casts of the ability (after the spirit has returned to Elder Titan and is summoned again).

Whether or not it is selected, the spirit (and all other currently controlled units) can still be moved along with Elder Titan by using CTRL+Right-click while "Unified Orders with CTRL" is turned on, but this does not actually select the spirit. I know that this doesn't actually select units in any other situation, I'm just listing it as a minor workaround.

Repro Steps:
  1. Turn on "Auto-select Summoned Units".
  2. As Elder Titan, level up Astral Spirit at least once so that you can use the ability.
  3. Use Astral Spirit, summoning the spirit.
  4. While the spirit is active, press the key that "Select All Controlled Units" is bound to.
  5. While the spirit is active, press the key that "Select All Other Units" is bound to.


Result:
  • The spirit is not selected when Astral Spirit is cast and "Auto-select Summoned Units" is turned on.
  • The spirit is not selected when using "Select All Controlled Units" while the spirit is active.
  • The spirit is not selected when using "Select All Other Units" while the spirit is active.


Expected Result:
  • Elder Titan's spirit is selected upon casting Astral Spirit when "Auto-select Summoned Units" is turned on.
  • After summoning the spirit, "Select All Controlled Units" selects Elder Titan, the spirit, and any other controlled units.
  • After summoning the spirit, "Select All Other Units" selects the spirit and any other controlled units.