Have now. Just noticed this on my U6+:
[2043880.286635] ieee80211 phy0: WA: free done event
[2043880.286635] 1022c280
[2043880.286635] len = 44
[2043880.286635] DW0 : 2c 00 07 30
[2043880.286635] DW1 : 04 99 b4 0e
[2043880.286635] DW2 : 00 80 01 89
[2043880.286635] DW3 : 00 00 00 41
[2043880.286635] DW4 : 12 80 ff 3f
[2043880.286635] DW5 : 00 00 00 41
[2043880.286635] DW6 : 13 00 0a 00
[2043880.286635] DW7 : 00 00 00 41
[2043880.286635] DW8 : 15 00 0b 00
[2043880.286635] DW9 : 00 00 00 41
[2043880.286635] DW10 : 17 00 0c 00
[2237026.460205] ieee80211 phy0: WA: free done event
[2237026.460205] 10224580
[2237026.460205] len = 24
[2237026.460205] DW0 : 18 00 03 30
[2237026.460205] DW1 : 01 39 24 66
[2237026.460205] DW2 : 00 40 01 90
[2237026.460205] DW3 : 03 c4 12 41
[2237026.460205] DW4 : 23 00 12 00
[2237026.460205] DW5 : b1 84 ff 3f
[2237952.147595] ieee80211 phy0: WA: free done event
[2237952.147595] 1022db80
[2237952.147595] len = 24
[2237952.147595] DW0 : 18 00 03 30
[2237952.147595] DW1 : 01 af 64 61
[2237952.147595] DW2 : 00 40 01 90
[2237952.147595] DW3 : 04 28 3f 41
[2237952.147595] DW4 : 24 00 13 00
[2237952.147595] DW5 : cb 8f ff 3f
[2237956.007201] ieee80211 phy0: WA: free done event
[2237956.007201] 1022b600
[2237956.007201] len = 24
[2237956.007201] DW0 : 18 00 03 30
[2237956.007201] DW1 : 01 dd 14 39
[2237956.007201] DW2 : 00 40 01 90
[2237956.007201] DW3 : 03 34 40 41
[2237956.007201] DW4 : 20 80 11 00
[2237956.007201] DW5 : 0c 90 ff 3f
[2237956.239859] ieee80211 phy0: WA: free done event
[2237956.239859] 1022bc40
[2237956.239859] len = 20
[2237956.239859] DW0 : 14 00 01 30
[2237956.239859] DW1 : 01 e6 84 56
[2237956.239859] DW2 : 00 c0 87 81
[2237956.239859] DW3 : c7 50 00 41
[2237956.239859] DW4 : 0a 80 ff 3f
This is a while ago now and I can't see anything wrong with any of the radios at the momemt. Can't say I noticed anything when it happened either, but I probably wouldn't if it's a temporary problem.
Interesting that this should happen with the 2.4GHz radio, which I'm only using for a few low speed IOT devices.