If your device is an EU device with full LM (lifetime maps) entitlement and your device has only a partial area of EU loaded (e.g. UK & ROI) you may not get reliable FM RDS/TMC reception. See 'note' in this link:
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Additionally, it seems that FM traffic is being generally degraded when a DAB or HD Radio traffic channel is alternately available reliably in an area - albeit that's anecdotally:
Users have reported issues with Garmin FM traffic in Europe, including problems with the traffic service not working or losing the traffic signal. Some have noted that the service had been discontinued, and others have experienced difficulties with the traffic information not being received. It is recommended to check for any updates or re-register the traffic subscription to ensure continued access to traffic information.