glitch on framing frame and art gallery

[Foxgirl Jasper]

so on both missions i have had a single security guard show up and be unkillable. He takes absolutely no damage no matter what type of weapon or grenades you use and he puts out damage like a bulldozer. has anyone else had this happen yet or is it just me? Framing frame has it happen on the third day and art gallery has it happen if you take too long after killing all the guards with SA

[Karma Jockey]

It's not a glitch. Just a counter-measure to discourage SA
You are doing it wrong. When the first reinforcement guard appears, dominate and answer the pager. As long as that reinforcement guard lives, additional (unkillable) guard wont spawn.

There's no reason to blame the game for limitations of your mods. Spycams, trip mines + HVT aced are your friend and make this heist (and most other heists) much easier.