Yeah I don't think its intentional, just how it interacts with the ability, as it changes to another ability for the duration, thus I imagine/I'm guessing it overrides/doesn't acknowledge the imbues the original ability has.
Which is unfortunate as the ability to imbue Sinclairs Ult would definitely be appreciated, although most people I imagine wouldn't imbue it at all beyond the compress cooldown which doesn't work properly to affect the cooldown until its upgraded to superior cooldown. (making it a useless purchase on the ultimate until you get superior cooldown, and better to imbue on something else before you can get the upgrade)
List of All (Standard, not legendary/brawl) Imbues and their Interactions (Loosely tested)
Ballistic Enchantment - Does not work.
Mystic Expansion - Does not work
Duration Extender - Does not work fully. It seems to effect how long you can hold the copied ult for before it expires, however it does not seem to effect how long the duration of ult effects lasts. Ie: Stun from Abrams ult does not last longer from extender.
Surge of Power - Does not work fully.
175 Abrams ult damage with surge of power, compared to 119 dmg with mystic expansion for equivalent spirit investment. 119 dmg sinclair copy, Sinclair gets the movement and fire rate effects, however not the damage increase from imbued spirit power.
Mystic Reverb - Does not work.
Compress Cooldown - Does not work
Quicksilver Reload - Does not work fully. The Reload works. However when doing damage with the ult, there is no extra damage.
Abrams deals 146 Damage using his ult with no upgrades and the only item being Quick Silver Reload, a 1.6k investment. He does 101 Damage using his ult if he has different spirit items (in my test case, ones that dont increase spirit from the item specifically like extra spirit but do increase investment - so 1.6k investment via mystic regen and spirit strike)
And Sinclair-Abrams ult deals 101 damage with Quick Silver Reload.
Mercurial Magnum - Same case: The Reload effect and bonus spirit damage on bullets does work, however the extra damage on the ult does not. 180 base for Abrams and then 247 total with the bonus damage added. Whereas it stays at 180 base for Sinclair.
With the Imbued Legendary items of Brawl, I remember someone trying frostbite charm on ult and it didn't work. Quick test shows it doesn't work. The other imbueable legendary, omnicharge pendant does not imbue on ultimate.