I didn’t see a way to edit my original post, but I ran tcpdump port 69 on the MaaS machine to view TFTP traffic, and found this after starting commissioning for a machine:
14:11:59.401800 IP aa.aa.aa.aa.2070 > bb.bb.bb.bb.tftp: TFTP, length 27, RRQ "pxelinux.0" octet tsize 0
14:11:59.407666 IP aa.aa.aa.aa.2071 > bb.bb.bb.bb.tftp: TFTP, length 32, RRQ "pxelinux.0" octet blksize 1456
14:11:59.655678 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 41, RRQ "ldlinux.c32" octet tsize 0 blksize 1408
14:11:59.696875 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 79, RRQ "pxelinux.cfg/44454c4c-4e00-1037-8044-b3c04f484b32" octet tsize 0 blksize 1408
14:11:59.706043 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.706196 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 63, RRQ "pxelinux.cfg/01-80-18-44-dd-9a-2c" octet tsize 0 blksize 1408
14:11:59.716057 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.716211 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 51, RRQ "pxelinux.cfg/0AF44341" octet tsize 0 blksize 1408
14:11:59.717194 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.717343 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 50, RRQ "pxelinux.cfg/0AF4434" octet tsize 0 blksize 1408
14:11:59.718851 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.718997 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 49, RRQ "pxelinux.cfg/0AF443" octet tsize 0 blksize 1408
14:11:59.720288 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.720440 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 48, RRQ "pxelinux.cfg/0AF44" octet tsize 0 blksize 1408
14:11:59.721707 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.721855 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 47, RRQ "pxelinux.cfg/0AF4" octet tsize 0 blksize 1408
14:11:59.723473 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.723598 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 46, RRQ "pxelinux.cfg/0AF" octet tsize 0 blksize 1408
14:11:59.725009 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.725133 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 45, RRQ "pxelinux.cfg/0A" octet tsize 0 blksize 1408
14:11:59.726523 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.726649 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 44, RRQ "pxelinux.cfg/0" octet tsize 0 blksize 1408
14:11:59.728291 IP bb.bb.bb.bb.tftp > aa.aa.aa.aa.49153: TFTP, length 19, ERROR ENOTFOUND "File not found"
14:11:59.728415 IP aa.aa.aa.aa.49153 > bb.bb.bb.bb.tftp: TFTP, length 50, RRQ "pxelinux.cfg/default" octet tsize 0 blksize 1408
Note that the aa.aa.aa.aa IP is the machine being commissioned, and the bb.bb.bb.bb IP is the machine running MaaS.
Are these file not found errors expected as part of the DHCP/PXE boot process?