Well I can chime in, 7 years later and hundreds of rolls later, the issue never returned for me. I cleaned the reel and tank thoroughly with warm water. At some point I switched from Flexicolor to Fuji chemicals.
@kyuut are you reusing your chemicals a lot? If the roll that got the bubbles very old? I had suspicions that expired film was made differently.
So, i re-use an replenish Bellini C-41 LR developer. i don't know exactly how many rolls i've run through this current working solution because i mostly develop ECN-2. my tank and reels were bought brand new and never had contact with photoflo or stabilizer, the machine is second-hand but periodically cleaned with citric acid.
i've had it happen with ECN-2 two or three times but i've done more than 300 rolls of ECN-2 film. i've run much less C-41 but the issue has been more persistent.
I posted on the Jobo groups but didn't get any useful replies, searching for old posts there and threads here didn't really get me anywhere either. I also chatted with a Mexican lab owner from the jobo FB group that has been replenishing for 10 years and he says it's normal for it to foam, but he doesn't have any issues even while running minimum volume.
my lab owner friend suggested that i should run the machine at it's limit (650ML) even with the multi tank 2. but the issue happened last night, i don't think this current batch has seen more than 30 rolls in a bit less than a month, since i discarded two batches due to contaminated film before
the only solution i've managed to find has been Edwal LFN but i'm still very resistant to putting a wetting agent in my developer and contaminating my ATL-1500.
I follow Z-131 instructions with a dry pre-heat, but i'm thinking about trying 2x1min 38c pre-wets as suggested by Photo engineer a few years back
I did two runs on a 2520 yesterday. a roll of fresh portra 400 had bubbles in the sky in one frame
the second run had an expired ferrania solaris and the issue was way more prevalent on this one. i got foam marks even in the last frame of the roll
edit: i do know kodak says not to replenish but after a lot of discussion with people in the business i've been repeatedly been told it's okay to do so. i'm not running strip tests because they are impossible to get but my results when scanning are fine. One-shotting the developer currently costs me more than my entire color process with replenishment.
if i can't find a solution i might look into making c-41 from scratch to potentially make one-shot viable cost-wise