This isn't a bug. It is a configuration setting, and it is one of the most frustrating experiences a spreadsheet user can face. You type a simple formula, hit Enter, and the text of the formula stares back at you, inert. The cell refuses to compute. The spreadsheet works fine otherwise, which makes the problem feel like a personal affront rather than a technical failure. We understand the impulse to call it a bug, but calling it that obscures the fix.
The behavior described, where a formula displays as plain text instead of executing, is almost always caused by one of two things. Either the cell is formatted as "Text" instead of "General" or "Number," or the workbook has been set to "Show Formulas" mode. In your case, given that you are using Microsoft 365 on both the app and online versions, the most likely culprit is the cell format. When you paste a formula into a cell that is pre-formatted as text, Excel treats the equals sign as a literal character. The application is not broken; it is following the instructions you gave it, even if you did not mean to give them. The copy-and-paste workaround you found works only when the source cell is formatted correctly, which is why it fails inconsistently.
The practical takeaway is simple and immediate. Select the cell or range that is misbehaving, go to the Home tab, and change the Number Format dropdown from "Text" to "General." Then press F2 to enter edit mode on the cell and hit Enter. Your formula will calculate. This is not a judgment on your skill level, it is a quirk of how Excel prioritizes formatting over function. The software is powerful, but it is not intuitive about this particular trap.
What this means for your workflow is that you can stop hunting for workarounds. The problem has a single, repeatable solution. You do not need to copy from other cells. You do not need to reinstall the application. You need to change the format and re-enter the formula. Once you know this, the frustration dissolves into a simple procedural step. That is the real value of understanding your tools: not avoiding every problem, but knowing exactly how to solve the ones that appear.