of course it can happen, even before the bull run happened the possibility of the top 20 position being taken can also happen, because if there is an altcoin it is better than the top 20, then of course the top 20 position will be taken, therefore if you want invest in cryptocurrency safely for the long term, then you better invest in Bitcoin, because Bitcoin position cannot be taken and will remain top 1 in cryptocurrency.
Depend on price stable or not make some coins on top 20 position will be change and their position drop or up regarding with transaction volume and price in lower or upper. There are not guarantee with 20 position top coins keep exist with their standings when get problem with their coins actually position will drop and out from top 20 position standings. Luna is top 10 standings coins but when price dropping drastically their position out from top 20 standings in CMC. Lets predict which one top 20 rank position standing coins in CMC will drop their position and replace by the other coins, will BNB loss their standings place after price dumping drastically from 300 dollar and today almost drop to 200 dollar.
Sometimes it does happen and those bad moments and situations do end up changing everything. When Luna crashed, it didn't just go down itself, it went down with everything else as well. This is why it's quite important to remember that you are not going to get any good result if you keep trusting the top names, they could crash at any moment.
I believe that there are way too many people who trust the coins we have right now at the top and I bet that when the bull run comes and then the bear market follows after that, 50% of the coins at the top 20 will go down from their ranks, not saying they will all go out of top 20, but they will be lower and some other new things will go up. This is the past, and it will be the future too.