I mildly recalls seeing a bug like this. The problem was in intel's kernel driver. A NIC by defaul checks incoming packets whether they match NIC's MAC. So if a TAP device was created over a NIC, it had to be put into promisc mode (automatically done by the driver) to allow different MACs and the check was done in kernel then. But since this generates too much interrupts, NICs HW was extended and it can be programmed with multiple MACs to let through. However, there was a bug which I recall of, that intel driver was not always putting the TAP MAC into the NIC HW correctly. Obviously, the bug was not visible if the NIC was put into promisc mode. And this may be what you are seeing. Let me see if I can find the bug.
That sounds like my problem even though i do not have an Intel nic.
03:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 10)
Cheers,
Stefan
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