PDF forms are lost after Save As...

I have a PDF document that has Fillable Forms that I want to use as a template.
Unfortunatelly there seems to be a bug in LibreOffice Draw that loses (or converts?) all fillable forms.
The new PDF documents no longer have fillable forms that can be seen in web browser.

Create the form in Writer. Then save as pdf.

Every time I try to open PDF document with Writer it automatically changes to Draw.

Anything able to print can print to a pdf file instead of paper. The other way round is like pushing the tooth paste back into the tube. LibreOffice opens pdf with Draw because pdf comes closest to a drawing.
Pdf can be opened with any pdf viewer on any device.

In a way you should treat the Writer-odt as “source-code” for the pdf. So you don’t open the pdf with LibreOffice. Instead LibreOffice generates the pdf from the .odt-source.
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Either you modify the source in Writer and (after saving the .odt) you export to pdf.
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Or you wish to fill the form with data. Then open the pdf with a pdf-viewer. Most are capable to fill forms today…
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As said by @Villeroy (and seen by you) the default part to modify a pdf is Draw. In some cases it may be useful to start Writer first, then search for an import-filter for pdf. But actually exporting to pdf (and even more printing to pdf) looses structure and informations from the .odt.
An option may be creating a hybid-file, where the pdf also includes its own source as attachment.

If I understand you correctly LibreOffice is unable to save PDF fillable forms in any format but .pdf format resulting in a document that can’t be reused or modified?

So I must start from scratch with Writer that is not really suitable for forms?

You don’t need to start from scratch. If you want to modify your form, modify the original document first. Then export to pdf. You do exactly the same thing when you export jpeg from your painting program, when you export mp3 from your sound editor, when you export mp4 from your video editor. There is one native file format supporting all the features of your editing application and there are widely spread export formats to be distributed as final product.
Same with word processing, desktop publishing, computer aided design, reporting software which all can produce pdf as print-out to file. This print-out can be opened with countless pdf viewers without being editable, unless you buy a license of Adobe Acrobat, which is the one and only program designed to edit all kinds of pdf, regardless of origin.

Draw is a work-around, good enough to fix some minor typos.
Then there are applications able to merge or split pages of pdf.
Most pdf viewers can fill out forms and attach annotations to pdf.

Hybrid documents is more than a work-around. LibreOffice can store pdf with its own document piggyback. When you open the pdf with LibreOffice, you get the original document for editing. Saving the modified document stores the hybrid pdf. Opening the same file with a pdf viewer shows a normal pdf.

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Here are a few tips from the wiki and guides:

FAQ-Draw - How to open PDF file in Draw

Create a fillable form in Writer

Content Controls in Writer

English documentation