Description:

Bought a mercenary stack from an independent city/tribe.

Mercenary stack of 2 units was moved onto one of my cities.

Recruited 2 Immortals whilst the mercenary stack was present.
Immortals were recruited to the mercenary stack and were then counted as part of the mercenary stack.
Could not remove the units from the mercenary stack.

Did not test If I would have lost the 2 units I purchased when the mercenary timer finished.

Unsure whether this happened due to unit recruitment beginning before the mercenary stack was present on the city. However, it did not block the 2nd unit being recruited/built whilst the mercenary stack was present.

Unfortunately do not have a screenshot or save game of this bug. Didn't think too much of it at the time unfortunately.