netwatch not working using an ipv6 link-local address

Both routers in this example are RB5009UG+S+ running 7.11.2

I’m trying to monitor connectivity over some interfaces using netwatch with an ipv6 link-local address as the destination:

/tool netwatch
add comment="Link to UPPER-1 via microwave" disabled=no host=fe80::4aa9:8aff:fe59:73b5%UPPER-1-microwave interval=10s name=UPPER-1-micro packet-count=5 packet-interval=1s type=icmp

The netwatch shows as down:

;;; Link to UPPER-1 via microwave
0 icmp  fe80::4aa9:8aff:fe59:73b5%UPPER-1-microwave  10s       down    2023-10-20 12:44:25

I can see the traffic over the UPPER-1-microwave interface:

[admin@LEYMOOR-1] [qui]> /tool/sniffer/quick interface=UPPER-1-microwave ip-protocol=icmpv6
Columns: INTERFACE, TIME, NUM, DIR, SRC-MAC, DST-MAC, SRC-ADDRESS, DST-ADDRESS, PROTOCOL, SIZE, CPU
INTERFACE          TIME   NUM  DIR  SRC-MAC            DST-MAC            SRC-ADDRESS                DST-ADDRESS                PROTOCOL     SIZE  CPU
UPPER-1-microwave  5.308    1  ->   48:A9:8A:59:6F:E1  48:A9:8A:59:73:B5  fe80::4aa9:8aff:fe59:6fe1  fe80::4aa9:8aff:fe59:73b5  ipv6:icmpv6    68    0
UPPER-1-microwave  5.309    2  <-   48:A9:8A:59:73:B5  48:A9:8A:59:6F:E1  fe80::4aa9:8aff:fe59:73b5  fe80::4aa9:8aff:fe59:6fe1  ipv6:icmpv6    68    2
UPPER-1-microwave  6.31     3  ->   48:A9:8A:59:6F:E1  48:A9:8A:59:73:B5  fe80::4aa9:8aff:fe59:6fe1  fe80::4aa9:8aff:fe59:73b5  ipv6:icmpv6    68    0
UPPER-1-microwave  6.311    4  <-   48:A9:8A:59:73:B5  48:A9:8A:59:6F:E1  fe80::4aa9:8aff:fe59:73b5  fe80::4aa9:8aff:fe59:6fe1  ipv6:icmpv6    68    2
UPPER-1-microwave  7.312    5  ->   48:A9:8A:59:6F:E1  48:A9:8A:59:73:B5  fe80::4aa9:8aff:fe59:6fe1  fe80::4aa9:8aff:fe59:73b5  ipv6:icmpv6    68    0
UPPER-1-microwave  7.313    6  <-   48:A9:8A:59:73:B5  48:A9:8A:59:6F:E1  fe80::4aa9:8aff:fe59:73b5  fe80::4aa9:8aff:fe59:6fe1  ipv6:icmpv6    68    2
UPPER-1-microwave  8.315    7  ->   48:A9:8A:59:6F:E1  48:A9:8A:59:73:B5  fe80::4aa9:8aff:fe59:6fe1  fe80::4aa9:8aff:fe59:73b5  ipv6:icmpv6    68    0
UPPER-1-microwave  8.316    8  <-   48:A9:8A:59:73:B5  48:A9:8A:59:6F:E1  fe80::4aa9:8aff:fe59:73b5  fe80::4aa9:8aff:fe59:6fe1  ipv6:icmpv6    68    2
UPPER-1-microwave  9.317    9  ->   48:A9:8A:59:6F:E1  48:A9:8A:59:73:B5  fe80::4aa9:8aff:fe59:6fe1  fe80::4aa9:8aff:fe59:73b5  ipv6:icmpv6    68    0
UPPER-1-microwave  9.318   10  <-   48:A9:8A:59:73:B5  48:A9:8A:59:6F:E1  fe80::4aa9:8aff:fe59:73b5  fe80::4aa9:8aff:fe59:6fe1  ipv6:icmpv6    68    2

And the debug logs show:

12:44:55 netwatch,debug [ UPPER-1-micro ] Stats:
12:44:55 netwatch,debug    [FAIL] rtt-max: 4294967.295 ms [ > 1000.000 ms ]
12:44:55 netwatch,debug    [FAIL] rtt-jitter: 4294967.295 ms [ > 1000.000 ms ]
12:44:55 netwatch,debug    [FAIL] rtt-avg: 4294967.295 ms [ > 100.000 ms ]
12:44:55 netwatch,debug    [FAIL] rtt-stdev: 4294967.295 ms [ > 250.000 ms ]
12:44:55 netwatch,debug    [ OK ] loss count: 5 [ <= 4294967295 ]
12:44:55 netwatch,debug    [FAIL] loss: 100.0% [ > 85.0% ]

However, if I run a /tool/ping with the same thing, it works!

[admin@LEYMOOR-1] /tool/netwatch> /tool/ping fe80::4aa9:8aff:fe59:73b5%UPPER-1-microwave count=5
  SEQ HOST                                     SIZE TTL TIME       STATUS
    0 fe80::4aa9:8aff:fe59:73b5                  56  64 1ms262us   echo reply
    1 fe80::4aa9:8aff:fe59:73b5                  56  64 1ms200us   echo reply
    2 fe80::4aa9:8aff:fe59:73b5                  56  64 1ms75us    echo reply
    3 fe80::4aa9:8aff:fe59:73b5                  56  64 1ms58us    echo reply
    4 fe80::4aa9:8aff:fe59:73b5                  56  64 1ms95us    echo reply
    sent=5 received=5 packet-loss=0% min-rtt=1ms58us avg-rtt=1ms138us max-rtt=1ms262us

Am I doing something wrong?