[BPI-R3] weird networking issue (and weirder "solution")

Banana Pi Router design BPI-R3/R3 Mini(MT7986)
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in fact it is sufficient to keep ‘Gbit switch B’. The other switch ‘Gbit switch A’ is no longer required. I now updated this in the above picture. I tried several switch brands for switch B. There is no difference. With

ethtool -K eth0 tso off
ethtool -K eth0 gro on
ethtool -K eth0 tx on

the issue reproduces almost instantly .

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Perhaps you can find a fix for it here:

Log - HEAD - openwrt/feeds/mtk-openwrt-feeds - Gitiles (mediatek.com)