Google Tag Manager is breaking src attribute in production

I have a tracking Iframe that is deployed through a custom HTML tag and triggered on the window loaded event.

The src attribute takes several custom JS variables to populate certain parameters where available.

However, in production the src attribute is full of errors that aren't showing up in preview mode. There are no issues with the variables themselves, as they have all been unit tested extensively.

Below are (1) the tag config for the iFrame's src attribute as shown within GTM, (2) the html produced in preview/debug mode, and (3) the html that's actually being produced in production:

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1. src="//servedby.flashtalking.com/container/14745;109435;11282;iframe/?ftXRef={{TransID Availability Test - Onetag}}&ftXValue={{Revenue Availability Test - Onetag}}&ftXType=FTXTYPE_VALUE&ftXName=FTXNAME_VALUE&ftXNumItems=FTXNUMITEMS_VALUE&ftXCurrency=FTXCURRENCY_VALUE&U1=U1_VALUE{{dataLayer Stations Output - Onetag}}{{Journey Date Output - Onetag}}{{Journey Ticket Type Output - Onetag}}&U8=U8_VALUE&U9=U9_VALUE&U10=U10_VALUE&U11=U11_VALUE&U12=U12_VALUE&U13=U13_VALUE&U14=U14_VALUE&U15=U15_VALUE&U16=U16_VALUE&U17=U17_VALUE&U18=U18_VALUE&U19=U19_VALUE&U20=U20_VALUE&ft_referrer={{Encoded Page URL}}&ns={{Referrer}}&cb={{Random Number}}"
2. https://servedby.flashtalking.com/container/14745;109435;11282;iframe/?ftXRef=2526968585&ftXValue=1.2***&ftXType=FTXTYPE_VALUE&ftXName=FTXNAME_VALUE&ftXNumItems=FTXNUMITEMS_VALUE&ftXCurrency=FTXCURRENCY_VALUE***&U1=U1_VALUE%26U2%3DU2_VALUE%26U3%3DU3_VALUE%26U4%3D28%2F12%2F19%26U5%3D%26U6%3DSDS%26U7%3DU7_VALUE&U8=U8_VALUE&U9=U9_VALUE&U10=U10_VALUE&U11=U11_VALUE&U12=U12_VALUE&U13=U13_VALUE&U14=U14_VALUE&U15=U15_VALUE&U16=U16_VALUE&U17=U17_VALUE&U18=U18_VALUE&U19=U19_VALUE&U20=U20_VALUE&ft_referrer=https%253A%252F%252Fwww.domain.co.uk%252Fbuytickets%252Fpaymentconfirmation.aspx&ns=https%3A%2F%2Fwww.domain.co.uk%2Fbuytickets%2Fpaymentconfirmation.aspx&cb=126603136
3. https://servedby.flashtalking.com/container/438;87804;9034;iframe/?ftXRef=2526968585&ftXValue=1.2&U1=paymentconfirmation&U3=1&U4=01000&U7=28/12/19&U9=BIRMINGHAM%20NEW%20STREET&U10=BIRMINGHAM%20SNOW%20HILL&ft_referrer=https%3A//www.domain.co.uk/buytickets/paymentconfirmation.aspx&ns=https%3A//www.domain.co.uk/buytickets/paymentconfirmation.aspx&cb=473400.6608119874

​​As you can see, the section in (2) in bold/italics is being removed entirely in production, despite the fact that these parameters aren't configured to be populated or modified by any variables. Similarly, parameters U9 and U10 are being populated seemingly from nothing - they aren't set to be populated by any variables.

Those parameters that are configured to be populated by custom variables aren't working properly - the values shown in production are either absent or else don't match the results given either in debug/preview or in unit testing, which suggests the problem is with GTM itself.

Are you sure that these are the same tag? Looking at the top tag it starts

>/container/14745;109435;11282

and the bottom tag starts
>/container/438;87804;9034

To me, the tags seem so completely different that I don't believe they're the same. GTM may do many things, but it doesn't randomly change your code! I've never seen it remove variables, or add them in, or change strings, it wouldn't have the ability to. Custom HTML tags are literally written 'as is' into the GTM.js file, with the exception of variables being replaced with macros.

If you've recently updated the tag then other reasons you're not seeing the changes may be:

1. You're looking at a cached version of GTM, in which case empty your cache, refresh the page and that should sort it.
2. You're working in and previewing changes that haven't been published, so won't show on the live site; if you publish the changes that should work.

If neither of those work then it would be good if you could share a link to the site so I can investigate!