It's not a display error. It's not the result of having multiple IOs slotted, I only have one. It's not the power in which I have the IO slotted (High Pain Tolerance, sentinel), I also have Numina's Convalescence +Recovery/Regeneration slotted in that power and it isn't stacking.
Every time I change maps, another 0.10% of Regeneration is added... and it's not limited to 5, either. The only constraints on this are the 120s duration of the +Regeneration and the player's map load speed. I just popped into and out of my base several times to make this screenshot, took me less than a minute to accumulate that much +Regeneration. For comparison, this character's total should be 3.74%. I have 15 stacks in this screenshot, and I could have stacked more.
I would've noticed this sooner, but I didn't have it slotted in the correct power on this character until my last couple of respecs (had it in Tough as a mule, never turned on, so the boost was never applied). Then I started seeing my total Regeneration varying, wildly at times, and woke up this morning with an urge to investigate.
Guessing that, when +Regeneration was added to the IO, it was flagged not to stack with other IOs, but not flagged not to stack with itself, like other +Regeneration boosts (Numina's, Regenerative Tissue). That would make it a legacy bug, but considering the exploitability of this, it really should be addressed.