To others who have described the same issue: You're describing exactly and precisely what I'm seeing.
To crinrict: I have seen your name many times when I search for answers here, and it is an honor to interact with you!
To begin with, I tried your test -- had a sim make garden salad downstairs while I remained upstairs. There was an unusually long gap of time before the money was subtracted for the cooking. My sims are in Riverside Roost; it should not have taken the sim that long to just descend the stairs and go into a kitchen almost directly opposite.
There were other delays during the cooking, but I can't tell whether they were part of the issue that I and others are describing.
At your discretion, you might want to test it with simply having them go up a staircase. Position a sim at some distance from the stairs, then switch your view to upstairs and ask the sim to come to a spot on the upper floor. See if s/he takes unusually long to get there and, in fact, ends up teleporting there, to wit. (I see some reports here of unusual "teleporting"; this may be the same Issue, but I'm unsure.)
Added note: This may not affect their own autonomous actions. My one little guy went downstairs to read at one point (bookworm) and when I went down about 15 minutes later, their time, he was placidly sitting at the table, reading the book.
The "glitch" never seems to affect a sim if you can see them; at one point, I told one sim to come upstairs and watched down the stairwell FROM the second floor. He ascended with no problem.
Answering your questions, the interactions don't drop from the queue, and to my knowledge, I don't switch sims. I say that not because I paid strict attention but because I find it hard to believe I immediately switch sims so often like that, LOL. But if you want me to test anything or try anything, I certainly can.
P.S. You replied fast!
