Checking all only. Very unique problem. Not the norm of an FX unless something is broken or off.
Have seen aquastop valve's break , part closed , causing a backwash can't hurt to check.
Actually water flow is : down inlet pipe all the way to bottom then up the outer circumference of the baskets/foam pre filter which then flows into the center part of the inter basket (your media setup) then all the way down the center to the center of bottom basket then directed by the molded guides into pump then back up the outlet pipe. It's water down , up , down , then up // [not down then up] if this is true for you then you are missing one of the tubes and reason for the problem. May of broken off and did not notice , things happen ? One tube is made into the lid and the other is made into the bottom casing. Was / is this a used filter can see why it was sold. If was a new filter then one tube is broken off to give you an down then up flow "only".
This is Fluval special flow pattern that make all FX's so special and efficient.
https://drive.wps.com/docs/1MT5VU2nOr your top center basket being nearly full of bio rings and purigen on top ( as you state) may not be giving the adequate clearance for the water coming up from the outer area to flow freely down the center causing a backwash. My manual states this , never overfill basket , especially top basket. Clearance is a must for flow to enter center part down. This would cause a backwash and filter will not work correctly. Sometimes we think more media will work better when a problem come's up , this filter design that's not true , It's an important note in the manual. Something is causing a backwash and or the pathway of the water is being redirected causing filter to not work the way it was designed. Like a missing tube. Only reason I said check all and you can google for the water path as I stated it is. You can find images of it and videos too.
Trying my best to explain better wish I was there hands on. A cracked/ broken basket lip , a dislodge pipe , missing pipe , seen some lose a pipe all together , yes it happens ... some find it in the yard or garden if lucky , broken molding on bottom something is causing disrupted flow so filter is not working properly. Wish I could help better ! Since we been through everything else it has to be in filter which can be corrected easy. That top basket is the main key having clearance for water to enter center and continue down for proper flow as noted in manual as important !
NOTE / NOTE / NOTE / PS / Ps : please read post on the 10th about tube alignment also very important !!! took me three edits just to add this....note // getting too dam old...