This is an old ss I took when diagnosing the problem and had other addons enabled, but I can recreate the issue with only Leatrix Plus and BigDebuffs enabled:

https://i.imgur.com/3ap4I0o.png

Essentially the character portrait frame strata is permanently higher than the BigDebuffs frame strata.

Updated ss showing stealth ability BigDebuffs icon on target frame but not player frame:

https://i.imgur.com/HhcCm6R.png

Let me know if you need more info, I'd guess this has to do with a hooksecurefunc in leatrix somewhere.

Nvm I was able to recreate the issue with only BigDebuffs enabled