Posting this in case it helps anyone else at some point. While dewinterizing my 2021 Tab 400, I noticed that the bathroom sink was not draining properly - water was flowing out of the crevice below the sink portion. Pulled off the front panel expecting to see a loose fitting but all looked fine. Then took a closer look at the crevice with flashlight in hand and noticed that the drain was completely covered with "gunk". Guessing it's four years' worth of toothpaste debris, etc. Ick!
But, a relatively easy fix. Managed to clear enough to confirm that this was the cause. Now, I'm just searching for a cleaning tool skinny and long enough to fit into that slot and get the rest of it cleared away.
Another item to add to my annual maintenance list - as well as note to self to not be so stingy with rinsing water.