PPI Royal Mail Large Letter Tracking ID not Valid

I Have a Royal Mail Click & Drop Business account, and use PPI to drop my parcels off in a sack at the Post Office.

I don’t use Royal Mail 48/24 as i’m not VAT registered, so 1st and 2nd class is cheaper.

When i send an item via 1st or 2nd class parcel (small or medium), i get a tracking number which i paste in to Amazon. This works fine.

When i send via 1st/2nd class large letter, i also get given a tracking number. Again, i paste this in to Amazon and select the delivery service as ‘Royal Mail 2nd Class’.

However, these large letter tracking numbers are coming up as Invalid by Amazon and affecting my Valid Tracking Rate. I can’t understand why!?

For example, an order placed on 20th March was posted on 21st March by Royal Mail 2nd Class Large Letter. Tracking ID: 32058960000046CE1DBD9

You can see by the tracking that this was delivered on 23rd March:
https://www.royalmail.com/track-your-item#/tracking-results/32058960000046CE1DBD9

Yet the Amazon report saying:
Has Tracking ID: Yes
Has Valid Tracking Number: No

Does anyone have any ideas how i can fix this?