undo tablespace

I have a rac instance w 2 nodes 19.3. my undo tablespace is getting very big it reaches 50GB ive noticed that most of the extents are in expired state but the undo management wont resue them instead its getting bigger. i tried the below :

1- setting the maxsize of undo datafile=actualsize 25GB with autoextend on. it worked for some time but the i hit the error unable to extend undo tablespace

2- As per oracle recommendation i set the "_smu_debug_mode" = 33554432 still the same issue happen