I have the same issue.
Have tried old and the latest openwrt firmware (19.07.0 - snapshot), with and without "switch" (my old WRT54GL and cisco switch), tried different periods of releasing the reset button, I have tried to update the stock firmware from 1.1.0 to 1.2.1, tried different machines (3 x Linux, 2 x Windows), different TFTP servers (atftp, tftp-hpa, dnsmasq, tftp64) none of it works.
As you mentioned, router actually detects the TFTP server. Tcpdump shows some sort of timeout:
21:15:38.207414 IP (tos 0x0, ttl 255, id 0, offset 0, flags [DF], proto UDP (17), length 73)
192.168.0.86.1469 > 192.168.0.66.69: [no cksum] 45 RRQ "ArcherC6v2_tp_recovery.bin" octet timeout 5
I also see some "link down | link up" entries in the log, so to me it seems like the router drops the connection.
I have also tried to flash the stock firmware via tftp, and the same thing happens, so I think there must be something wrong with the router itself. I don't think it's some sort of validation issue, since firmware is never downloaded.
Does anyone know of a way to connect (telnet, ssh...) to router and see boot logs?