I've also been having this issue! In a Residential Rental, playing as a tenant. (I've only had it happen in one particular save). I tried demolishing and rebuilding the bathroom walls, but that didn't help. My other sims won't use the surrounding rooms if a sim is in the shower.
Originally had the issue with the Almost InvisiShower 2.0 from Discover University, so I replaced it with the Steamy Times shower from Snowy Escape and there's still an issue, even though that shower should be fine to use while in a shared space. I wondered if it was a glitch with that style of shower? The Residential Rental my sim is in has a second floor above, with other tenants, so it should be fully enclosed/considered having a roof.
For reference: Issue was found on Mac, and I am only using MCCC mod (up to date).
I also opened a fresh save, placed two Steamy Times showers next to each other and had sims use these without issue.
