I had a similar situation a few years ago. The conclusions I reached from the regs and code were that the refund does get reclassified as catch-up as of the end of the plan year, but that does not reduce the amount the participant can defer for the calendar year. Our valuation system treated it the same way. Using your numbers:
$6,500 of the refund is reclassified as catch-up as of 4/30/20. The plan's determination of catch-up does not affect the participant's maximum deferrals [see 402(g)(1)(C)]. So, the catch-up eligible participant can still defer $26,000 for calendar year 2020. If the catch-up eligible participant defers $26,000 from 5/1/20 - 12/31/20, all $26,000 of it counts in the 4/30/21 ADP test since the plan has already used the entire $6,500 of 2020 catch-up for that participant as of 4/30/2020.