1. check for a dented trans pan.
2. Add a bottle of Auto Trans-X to the ATF, available at any auto store or Walmart not that I encourage supporting that low grade cheap-stuff-addiction machine.
*** Keep in mind additives are horrible ideas most of the time and should be avoided. There are a *very tiny* percentage of situations where additives can address a very specific make/model/part/location issue with reasonable efficacy. 1999 automatics are one of those rare cases - like about the only (and most common) one in Subaru world. Anyway - don’t extrapolate that these money extracting desperation bottles are good ideas. Avoid reading the marketing drenched labels that prey on your desperation
This issue has been present too long, it should have been addressed awhile ago. Maybe add a second bottle if the first doesn’t help.
Changing the fluid is a good idea if it’s never been done. But that’s not going to fix this