Yate follows the ID3 specification for Involved People and Musician credits.

The field consists of multiple lines specified as: instrument=musician{,musician}.... When the data is saved to files, empty lines and unnecessary whitespace is automatically removed.

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It's next to impossible to do in a TSV file as there is no way to express the newline characters. You can do it in a properly encoded CSV file. If you really have to use TSV use any sequence you wish to replace the newline characters. For example ;;; as you did in your example. After importing use the Multi Field Editor or an action to replace the ;;; with a newline. If you use the MFE you'll have to use a regular expression for the replace as you need the \n escape sequence. In an action you can do a simple text replace.

The attached screenshot shows both.

If you created a TSV via an action you would have received a warning saying that you have to use an alternate multi line delimiter.

The three steps that you performed taken by themselves are correct. The entire process does not guarantee the correct formatting of the data. In this case the Musician Credits data in the TSV file is not correct. However you repopulated the TSV you ended up with:

Drums=Lee Carey;;;Bass=Nic Dalton;;;Violin=Eleanor Gray;;;Cello=Simeon Johnson

It's just not the correct encoding.

There's a number of things which can be done. The simplest is to replace the Using 'Apply database values to open files' from the context menu by an action which you can run from the same context menu. The action will do the apply and clean up Musician Credits. This way you can leave the encoding by slightly changing the workflow.

I'll gladly write the action for you. Go to Settings - Export Sets and export your Export Set as a zip file. I probably can do without it but it will be a little easier if I see what you're exporting. Email the zip to support@2manyrobots.com

For anyone else reading this post ... an action was supplied. As it does a few other things as well it has not been posted. If anyone is interested, reach out and the action will be supplied.