I have a bunch of MikroTik 750gr3 routers on which I install OpenWrt. One of them was in reserve and still had RouterOS on it, and just now I wanted to install OpenWrt on it, so I did the TFTP dance to boot it using openwrt-23.05.2-ramips-mt7621-mikrotik_routerboard-750gr3-initramfs-kernel.bin. On the TFTP host the output showed dnsmasq-tftp: sent /tmp/openwrt.bin to 192.168.x.x, the router beeped and all that, but trying to ping it (after moving cable to LAN port) sparked no joy. Rinsed and repeated various times without luck, tried it on another 750gr3 (one that already had OpenWrt on it) with same results. Then I went through the whole dance with openwrt-21.02.7-ramips-mt7621-mikrotik_routerboard-750gr3-initramfs-kernel.bin with still identical results.
So then I tried openwrt-21.02.7-ramips-mt7621-mikrotik_routerboard-750gr3-initramfs-kernel.bin and lo! ping showed success. But no Luci on the HTTP port and after ssh-ing into the router I saw what looked like an incompletely booted system. Still, this was somewhat encouraging, so next I tried openwrt-22.03.1-ramips-mt7621-mikrotik_routerboard-750gr3-initramfs-kernel.bin openwrt.bin and this time I had a fully working OpenWrt booted and was able to use Luci to flash the latest image.
So, my problem is solved, but any ideas why this might be happening?