Hm, this seems to be independent of the external mouse.
Just tried to connect another bluetooth mouse (I believe not LE) which doesn’t work neither. Both mice connect without problems to my Librem14 (using an external ASUS BL adapter, because the internal bluetooth does not work with a headset of mine).
Librem14:
root@PureBlackSoul:~# hcitool info 00:60:D1:00:04:F9
Requesting information ...
BD Address: 00:60:D1:xx:xx:xx
OUI Company: CASCADE COMMUNICATIONS (00-60-D1)
Device Name: Bluetooth Mouse
LMP Version: 2.0 (0x3) LMP Subversion: 0x102
Manufacturer: Syntronix Corporation (56)
Features: 0xaf 0xfe 0x09 0x38 0x18 0x18 0x00 0x00
<3-slot packets> <5-slot packets> <encryption> <slot offset>
<role switch> <sniff mode> <RSSI> <channel quality> <SCO link>
<HV2 packets> <HV3 packets> <u-law log> <A-law log> <CVSD>
<transparent SCO> <enhanced iscan> <interlaced iscan>
<interlaced pscan> <AFH cap. slave> <AFH class. slave>
<AFH cap. master> <AFH class. master>
Librem5:
root@pureos:/lib/firmware# hcitool scan
Scanning ...
00:60:D1:xx:xx:xx n/a
root@pureos:/lib/firmware# hcitool info 00:60:D1:xx:xx:xx
Requesting information ...
Can't create connection: Input/output error
The Librem5 finds the mouse in scans only occasionally.