Why do you think that would that be your best solution?
You are correct, your mash efficiency doesn't change because you lost more trub and when you don't adjust your brewhouse efficiency to make up for the lower brewhouse efficiency higher losses causes BeerSmith boosts something to make up for you having the wrong brewhouse efficiency entered and that's an increase in mash efficiency.
I just put a BIAB pretend recipe in.
70% brewhouse, 6 gallon batch, half gallon trub loss, est OG 1.049, est mash eff 75.8%.
Oh no, I lost an extra half gallon to trub. Adjust batch size down, est OG goes 1.053. Adjust trub loss without changing brewhouse efficiency, OG stays 1.053 but mash efficiency goes to 82.7%.
Adjust brewhouse efficiency down 6% to account for the extra trub loss, OG goes to 1.049 where it was and est mash eff goes back to 75.6% where it originally was.
I end up with 64% brewhouse, 5.5 gallon batch, 1 gallon trub loss, est OG 1.049, est mash eff 75.6%.