How does the NVRAM value look on your RT-AC68U? Low NVRAM can lead to weird issues. Lots of recent past discussion on the low NVRAM issue, with the RT-AC68U running the last few Asus-Merlin firmware versions, that one can find using the forum search.
If one is having low NVRAM issues the first step is to do a hard factory reset followed by manual reconfiguration (do not load a saved CFG file post hard factory reset). From there, if one is using a lot of manual IP reservations, using YazDHCP might help free up some NVRAM. Beyond that there are some additional scripts and lines of code one can run to further free up some additional NVRAM, but those scripts and lines of code do not survive reboot. One can also flash the 386.11 Alpha but that carries the risk of it being alpha firmware and not release firmware. The 386.11 Alpha has some tweaks by RMerlin to try and address the low NVRAM issue.