I'm currently using Bizhawk 2.9.1 and every time I do attempts, swimsuit is in the same closet each time. Wanted to check whether this is a potential issue?

Unless it is really good RNG or you found a manipulation. I haven't heard of this happening on either console or emulator.

It's normal for the RNG to be fixed if you're starting from a hard reset. It also works the same on Master System hardware, since the BIOS puts the RAM into a known state. This will guarantee the same sequence of swimsuit locations from game to game. Note, swimsuit location is set on game load which happens on normal start OR when the demo starts after the title screen.

Running with SMS BIOS 1.3 should give the best swimsuit RNG if you're starting from a hard reset. In other cases, you may have to manually cycle the RNG by letting the demo load a number of times before starting. Exception: if you're running on hardware without a BIOS (e.g., Genesis w/ Power Base Converter), you can't guarantee the RNG because RAM doesn't get initialized.

So for original console players, you're saying the way to manipulate swimsuit RNG is to just start the game from a hard reset without letting the demo play through?

It depends on the BIOS version, BUT, for an original US/Europen Master System with the v1.3 BIOS, yes that will work. In other case, you may have to do some testing to determine the number of demo iterations needed. Regardless, you should be finding the swimsuit in the same location after hard reset, and that location should change predictably each time the demo loads.