I know what you're going through but I do not have a solution. I've used Convatec drainable pouches all of my life. For 9 years, I had no issues with the tape that anchors down the wafer.
On the 10th year, I had the itchiness just under the tape and my skin would weep yellow serous fluid, stain the tape and the tape would slough off. The itchiness was so bad, my ears would ring. I was told by my ET nurse to check with the manufacturer to see if there was a change in the ingredients on the adhesive for the tape. But that had no results.
I then spoke to an allergist and asked the same question. You CAN develope allergies over time. And that seemed to be the case for me. I then just started cutting the tape off of the wafer and I haven't had an issue since.
I know there have been other posts about this and other people have different solutions. Might want to check those out.