Before anyone again shoots the messenger, please consider that none of what follows represents critique or criticism - but is instead intended to provide solid information.
A software update is generally intended to bring new features (and with it, by definition, change). Some updates will not introduce change - but will be intended to offer a fix (otherwise known as a “patch”) for something that doesn’t work as intended. So, if you see an update, you should expect to see potential change.
You can get a good idea as to the purpose of an “update” by looking at the release notes or official documentation; the onus is on you to check. Apple are very good in providing comprehensive documentation - and if you don’t take the initiative to use or review provided resources, it is inevitable that you’ll encounter “unexpected changes”.
Another place from which to derive the purpose of an update - is the version number. Here I’m only going to illustrate Apple’s iOS/iPadOS numbering as it is this that is relevant to the discussion...
The version number is in three parts; taking the current release as an example, 13.4.1:
13 - represents the major version number. This will be a major release, in great probability, will offer major changes over the previous major release (in this case, the previous release was iOS12).
Big change - new features and functionality - big differences and change.
.4 - represents a feature update. The incremental update from “.3” to “.4” will have brought additional features and changes within both the feature-set and user interface (UI). This incremental update will include “patches” for the prior release.
New features - anticipate changes.
.1 - represents a patch that fixes either a functional issue or security vulnerability. By and large, no new features or substantial changes are likely to be present - unless the change is, of itself, a fix for something that was previously broken. The next “patch” is very likely to be e “.2”.