Popup cycleway suggestions layer does not appear

[si-the-pie]

Problem

Screen snapshot from Firefox 86.0 on MacOS 14.

Reproduce

Going to e.g. this region of London:
https://bikedata.cyclestreets.net/popupcycleways/#11.07/51.4918/-0.0613
and activating the Popup cycleway suggestions layer shows 77 features were found but they are not shown on the map and this appears in the console:

Error: layers.popupcycleways_linestring.paint.line-width[3][3][3][3]: 'undefined' value invalid. Use null instead.

API call

The api call appears to produce valid data:

https://api.cyclestreets.net/v2/advocacydata.popupcycleways.suggested?bbox=-0.248008%2C51.432334%2C0.125362%2C51.551107