It looks like the new version of fmt breaks waybar for Sway users. fmt 7.1.0-1 introduces this error:
waybar: symbol lookup error: /usr/lib/libspdlog.so.1: undefined symbol: _ZN3fmt2v76detail10vformat_toIcEENS0_20basic_format_contextISt20back_insert_iteratorINS1_6bufferIT_EEES6_E8iteratorERS7_NS0_17basic_string_viewIS6_EENS0_17basic_format_argsINS3_IS4_INS5_INS0_13type_identityIS6_E4typeEEEESH_EEEE
This looks like it is inherited from Arch, as users there have been experiencing this for a few days. Downgrading to fmt 7.0.3-1 solves the problem for now.
EDIT: It looks like Arch pushed out a fixed version of Waybar recompiled against the new fmt. Is there a chance that we could get this pushed out to Testing?