It’s not a bug when systems are overlapping. It’s certainly not convenient, but it’s a limitation of Dorico’s automatic casting-off system that sometimes needs a hand. Daniel has already taken the time to explain how this works in detail (you should find the explanations with a good search on this forum*). Dorico decides whether those systems can fit a page before it considers other stuff, like text markings, rehearsal marks… that take some vertical spacing, so either you force it to use only one system per page there (using a frame break) or you decide you can use a smaller space size on this page to make those two systems fit comfortably, and you use the break’s property in Engrave mode to change that size for this page (which means you create a frame break at the top of this page, another at the top of next page, first break gets a new value for space size, second break reverts to the value chosen in Layout options>Page setup). That’s some stuff Dorico cannot do for you, and since it’s like this by design, it’s not a bug, but a limitation.
- for instance here.