Has anyone ever encountered a "make up provision" in an Earn Out?

Agreed with the previous comments, Not uncommon on multi-year earnouts, particularly as you are coming out of COVID, and companies are discussing if you use 2019, 2020 or 2021 as a "normal" year, sellers will be sensitive that things can happen in a single year that may not necessarily affect the direction of the company.
Also relevant in businesses with large contracts where you might get a big contract that for one reason or another gets shifted to january rather than december and so you need to account for it in the following year.

During my time in Private equity, we used a "make-up provision" pretty regularly for lower-middle market deals. Whereby, if the earn-out is multi-year then the seller has an opportunity to "make-up" for a missed earn-out target in one year, if the target is then overachieved the following year. For example, if the earn-out is paid out over 2 years and the target is achieving $50mm in revenue in each year, but the first year the achieved revenue is only $45mm but the second year is $55mm then the seller would be made whole as part of the second payment. In these cases, it was mostly used as a negotiation tool if the seller was uneasy about the earn-out, in general.