I deleted my previous post becasue right after posting I took the time to triple check.
I narrowed down the workstation which when turned on generates the issue by asking people who was having problems with which workstation. I turned it on and as soon as it turned on I saw this in the logread output of the OpenWrt router:
Mon Aug 16 12:22:54 2021 kern.info kernel: [30154.168417] mt7530 mdio-bus:1f lan1: Link is Up - 100Mbps/Full - flow control off
Mon Aug 16 12:22:54 2021 kern.info kernel: [30154.183417] br-lan: port 1(lan1) entered blocking state
Mon Aug 16 12:22:54 2021 kern.info kernel: [30154.193904] br-lan: port 1(lan1) entered listening state
Mon Aug 16 12:22:54 2021 daemon.notice netifd: Network device 'lan1' link is up
Mon Aug 16 12:22:59 2021 kern.info kernel: [30158.327955] br-lan: port 1(lan1) entered learning state
Mon Aug 16 12:23:03 2021 kern.info kernel: [30162.424015] br-lan: port 1(lan1) entered forwarding state
Mon Aug 16 12:23:03 2021 kern.info kernel: [30162.434797] br-lan: topology change detected, propagating
Mon Aug 16 12:23:09 2021 kern.info kernel: [30168.504362] mt7530 mdio-bus:1f lan1: Link is Down
Mon Aug 16 12:23:09 2021 kern.info kernel: [30168.514147] br-lan: port 1(lan1) entered disabled state
Mon Aug 16 12:23:09 2021 daemon.notice netifd: Network device 'lan1' link is down
Mon Aug 16 12:23:11 2021 kern.info kernel: [30170.552573] mt7530 mdio-bus:1f lan1: Link is Up - 100Mbps/Full - flow control off
Mon Aug 16 12:23:11 2021 kern.info kernel: [30170.568040] br-lan: port 1(lan1) entered blocking state
Mon Aug 16 12:23:11 2021 kern.info kernel: [30170.578533] br-lan: port 1(lan1) entered listening state
Mon Aug 16 12:23:11 2021 daemon.notice netifd: Network device 'lan1' link is up
Mon Aug 16 12:23:15 2021 kern.info kernel: [30174.712372] br-lan: port 1(lan1) entered learning state
Mon Aug 16 12:23:19 2021 kern.info kernel: [30178.808251] br-lan: port 1(lan1) entered forwarding state
Mon Aug 16 12:23:19 2021 kern.info kernel: [30178.819063] br-lan: topology change detected, propagating
I guess br-lan: topology change detected, propagating is normal, isn't it? The LAN topology is changing because a new node is online, but mt7530 mdio-bus:1f lan1: Link is Down and then up in a matter of a second doesn't look ok.