Did you verify a checksum in case the download got corrupted?
A good point, I didn't. I don't have access to the router right now, but I've just re-downloaded the image from here. The file hasn't changed since it was built on May 17th. If I try to calc the checksum:
sha256sum -b openwrt-19.07.3-ipq40xx-generic-glinet_gl-b1300-squashfs-sysupgrade.bin
It produces the same 26edf2941b0a060766f0c5c4adf6a92da2fa00d89d2717948abfb19788d6246a hash as in the download folder, but is still different from the picture in my original post. I'll try again as soon as I get to the router.
Updated, I flashed the factory firmware qsdk-b1300-3.027.img via UBoot Web UI, verified the basics of the router are OK, then flashed OpenWrt v19.07.2 via UBoot TFTP, then upgraded to v19.07.3 via luci sysupgrade. This time, the hashes did match and I didn't not have any issue. I conclude the original glitch must have been something very specific to my router at that time.