m/z=32 but no m/z=28

Hello, I've been troubleshooting GCMS for a series of peaks that come out before general solvent peaks - at very early retention time.
Please see the image1 below. MS spectra of those peaks show m/z=14, 28, 32 for 1st,2nd peaks, then m/z=18 for third and then m/z = 18, 31, 44 for the 4th peak, which indicates, N2/O2, H2O and then MeOH, CO2. The largest peak is acetone.
I have opened inlet assembly, cleaned it, replaced liner septum and trimmed the column.
Air water check passed. But I still see those peaks after the cleaning though the intensity reduced. But what's even more strange is on the next day, when I ran a blank acetone, I see the peaks again and now MS spectra show only m/z=32 with no m/z=28 for the first two peaks (see image 2). The third peak shows m/z=31 and 44 which didn't change from before (see image 3). I'm curious how the ms detection has changed although the GC chromatogram looks comparable to each other. Air-water check has passed. If this is a common issue and there is any information available, please let me know. thank you

image 1

image 2

image 3

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