TL;DR: 10 Gbps of bad traffic at the time they took it over, but the main issue seems to have been that many networks were unable to reach the IP due to misconfigured equipment on the route.
If the misdirected traffic was 50 Mbps, I would expect Cloudflare to be able to simply ignore it. (Edit: it was 10 Gbps in 2018 when Cloudflare took over, which I expect to be an annoyance but not a problem if spread over multiple internet exchanges).
Given the number of people now relying on Clouflare 1.1.1.1 to "get Internet" (ie using 1.1.1.1 as recursive name server), I can't imagine APNIC deciding to stop Clouflare using this range.
It seems "too late" to revert this decision. Otherwise people will experience "Internet stopped working", blaming their ISP.
APNIC may decide to keep a working DNS server on 1.1.1.1, but ethically, routing traffic to someone else than Cloudflare is not great.
If just temporary assigned to Cloudflare, APNIC shouldn't care if it sees a better use for the range. Supporting unintended uses only encourages various types of abuse. And changing DNS settings is easy enough.
That said, if a lot of people rely on 1.1.1.1 as DNS, it's worth considering whether reassignment qualifies as 'better' use of this resource. Not to mention the hassle caused by making changes to popular [anything].
Fixed IP addresses change and are deprecated all the time. It's of zero concern to APNIC that customers of Cloudflare or various ISPs can no longer access the internet because they relied on a temporary IP assignment, after the service was gracefully terminated and deprecated, with ample lead time.
That being said, the use by Cloudflare is an excellent way to reclaim this part of the IP space, I don't see why they would terminate this collaboration.
