AMS breaks equation positioning

[Jack Henry]

Steps to produce the issue:
Performing the following in LyX:

1. Produce the pdf of Chapter 3. All the equations should be centered in the page
2. Place the cursor below the first equation in Chapter 3
3. Insert > Math > AMS * Environment (any AMS environment)
4. Produce the pdf. All the equations become aligned to the left of the page

Alternate Method:

1. Produce the pdf of Chapter 3. All the equations should be centered in the page
2. Right click an equation in Chapter 3 LyX
3. AMS Environment > click and environment
4. Produce the pdf. All the equations become aligned to the left of the page

Expected Output:
The equations should be centred to the page. I found this issue when trying to make a multiline equation to display a long equation broken between two lines.

Product Versions:
Windows 10 64 bit
MiKTeX Console 4.2.1
LyX 2.3.6 or 2.3.0or2 (both seem to produce the same issue)
Classic Thesis v4.4 or v4.6 (both seem to produce the same issue)

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Someone I know submitted using Classic Thesis in 2018. I tried using their version of LyX (LyX 2.3.0or2)

[Ivo Pletikosić]

For some reason LyX adds \PassOptionsToPackage{fleqn}{amsmath} to the preamble when the amsmath package is loaded. It should be enough to add \@fleqnfalse to Document Settings → LaTeX preamble in LyX to undo the fleqn option.

[Jack Henry]

This has indeed fixed the problem. Thank you so much for the quick response

[Ivo Pletikosić]

You're welcome, Jack. Happy to help!

And please: https://www.miede.de/postcards/ ;)

[André Miede]

Thanks a lot, Ivo, for quickly and successfully fixing this.

[André Miede]

See pinned answer, please.