After my Sims go on maternity leave, their job instantly goes to career level 10.
I'm not sure but was there a mod to fix this?
For which game is this happening and what EPs/SPs, etc, do you have besides the base game, if any?
If this is happening in Sims2, then it is not normal or default behavior and you probably have some mod which is causing it to happen. Remove your mods and see if it recurs. Then you can slowly add your mods back in and test (quit without saving so you can keep testing) until you determine which mod is the culprit.
I cannot speak to other versions of Sims games.