Not reading all characters after seek

I'm trying to make a program that inserts log entries into a text file. The issue I'm having is that I read through the file line by line for a specific line and want to write before the line. Python correctly reads the line I'm looking for, however, when I seek to go back to the previous position, it does not read the entire line anymore. I checked the offset and it's exactly the same but for some reason the entire line isn't getting read.

def fileWriter():
    immediateTrigger = 0
    returnTrigger = 0;
    with open('C:\\testData.txt', 'r+') as file:
        for line in iter(file.readline, ''):
            #line = file.readline()
            if 'Beginning of text entries' in line:
                print('arrived at text entries')
                print(file.tell())
                print(line)
                immediateTrigger = 1
                file_pos = file.tell()
            while not line.strip() and immediateTrigger == 1:
                #print('here')
                prev_pos = file.tell()
                print(str(prev_pos) + 'before')
                newLine = file.readline()
                print(newLine)
                if 'Text Entry 25' in newLine:
                    file.seek(prev_pos)
                    print(str(file.tell()) + 'after')
                    print(file.readline()')
                    immediateTrigger = 0

The output I would get is:

arrived at text entries
Text Entry 1: The 1st revision
(random entries...)
36800 before
Text Entry 25: The 25th revision
36800 after
try 25: The 25th revision

Why does it cut off like this?

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