Next time you can see if the issue is present on the development version of the website which is
https://mempool.ninja, it is made by the people behind mempool.space as a testing ground for new changes, so it is possible that fixes appear here first, before they are pushed to the main site.
It was definetely their node's issue. Personally I run mempool explorer on my node and it worked perfectly during that time. So it's not a problem with the app, but rather with the underlying node.
It is difficult to see how a bitcoin node could miss so many blocks from peers unless it was a network issue, in which case it would make perfect sense.