Hi. I just tried it in my Illustrator 2025 (v. 29.0) on macOS and verified the color with a couple of non-Adobe color samplers. The #5aaec7 worked great.
- Is your document in RGB or CMYK? You can check from File > Document Color Mode. Make sure it's RGB.
- When you say "I keep getting... #58adc7", how are you getting that? Is Illustrator showing the number changing in the Color panel, or is some other application showing the result of #5aaec7 used in Illustrator as coming out to be #58adc7?
- What is your version of Illustrator and operating system?
- If you quit Illustrator, start it up again, and try with a brand new RGB color space document, do you still get color shifting?