Commercial flights flying unusually low

[F-MANU]

Several A/C with very low cruise levels (below FL200) seen in french airspace over the last days:

Today's FR24 capture:

1890

A consequence of ongoing ATC strikes ? SN3670 (A319) TLS-BRU on 8th march even cruising at FL120 !! the fuel penalty must be :ugh:

What is the rationale behind such severe limitations ?

Have a nice day !

[atakacs]

Could it be that they went VFR?

[WindSheer]

Airspace is split into sectors, including levels.
I assume that the slot restrictions in the uppers were severe due to limited atc coverage, but the lower were OK so the flights were filed for the lower levels.

Perfectly OK, just longer flight and more fuel burn.

[student88]

Flying lower allows airlines to avoid slot regulations which may be imposed on higher sectors. On occasion Ops will ACARS/call us to ask if we are able to accept a different level on a subsequent sector to avoid a CTOT delay. Alternatives can include massive reroutes such as totally avoiding French airspace e.g via Brussels or the Tango routes for Spanish/North African destinations.