This is a problem for my Sims as well. Sims simply will not complete their yoga routines, no matter which routine I select. The game knows it is interrupted because they do not get the buff when they stop. Typically they interrupt their routine with a computer activity (programming, chat, game), but Sims who are locked from the computer will also interrupt a routine to do other things. Sims put their shoes back on and stay in their fitness outfit. To find this bug, you should be able to ask a Sim to complete a yoga routine, then wait for them to get through a few poses. They will decide to do something else.
This is a somewhat recent bug. I bought the Snowy Escape pack 12/22/2020. I wanted my yoga-loving single mother Sim to enjoy morning yoga routines during her mountain vacation while her daughter slept in but she kept interrupting the activity. I thought it was vacation excitement but, since then, any Sim won't finish yoga. I do not remember whether this was an issue between the 12/14/2020 update and my purchase of Snowy Escape.
The bug was It was not fixed in the 1/21/2021 patch. I do not have mods.
