Bitcoin - why do some transactions not get into the block, even with any commission?

Question

bitcoin

For the experiment, I made a fork of the cue ball. Transactions behave very strangely - when the norms work, when they do not fall into the block at all. Everything is fine with the commissions, I did not find any dependence on the size of the commissions. It just hits the block, sometimes it doesn't. They hang in the mempool and that's it. What could be the problem?.
Still very strange - for example, I create a transaction - it does not fall into the block for several hours (mining is stable), I check the box "Spend unconfirmed change" and create a new transaction in the inputs of which there is an input of the previous transaction - the transaction falls into the block, and which not in the block (hanging in the mempool) - this is lol. What the?