[BUG-REPORT] join(): rprefix not systematically taken into account

Description Conducting join operations in a loop, while using rprefix parameter, I noticed that this parameter is not systematically used, always for the 2nd iteration of the loop. Why is that so?

Minimal Reproducible Example

import numpy as np
import vaex as vx

n_cols = 3
n_rows = 3
n_df = 3

# Generate test data: a list of vaex DataFrames, for joining in a 2nd step.
index = np.arange(n_rows)
x = np.ones(n_rows)
vdfs = []
for k in range(1,n_df+1):
    vdf = vx.from_arrays(**{f'x_{i}': x*i+1 for i in range(n_cols)},
                         timestamp=index)
    vdfs.append(vdf)

# Join with vaex.
left = None
for i, vdf in enumerate(vdfs):
    rprefix = f'vdf_{i}_'
    if i < 3:
        print(rprefix)
    try:
        left = left.join(vdf, on='timestamp', rprefix=rprefix, how='inner')\
                   .drop(rprefix+'timestamp')
        if i < 3:
            print(left.get_column_names())
            print('')
    except AttributeError:
        left = vdf
        for col in left.get_column_names(regex='^(?!timestamp)'):
            left.rename(col, rprefix+col)
        print(left.get_column_names())
        print('')

The print statements displays rprefix and the resulting column names as the vaex DataFrame left gets new columns, only for the first 3 iterations. It displays:

vdf_0_
['vdf_0_x_0', 'vdf_0_x_1', 'vdf_0_x_2', 'timestamp']

vdf_1_
['vdf_0_x_0', 'vdf_0_x_1', 'vdf_0_x_2', 'timestamp', 'x_0', 'x_1', 'x_2']

vdf_2_
['vdf_0_x_0', 'vdf_0_x_1', 'vdf_0_x_2', 'timestamp', 'x_0', 'x_1', 'x_2', 'vdf_2_x_0', 'vdf_2_x_1', 'vdf_2_x_2']

The trouble is the 2nd paragraph (2nd iteration). You can read 'x_0', 'x_1', 'x_2', which shows that rprefix is not used for this 2nd iteration. Expected column names should be 'vdf_1_x_0', 'vdf_1_x_1', 'vdf_1_x_2'.

We can see that with the 3rd iteration, rprefix is used ok. (and so for the following iterations)

Please, is this a bug?

Software information

Vaex version: {'vaex-core': '4.5.1', 'vaex-hdf5': '0.10.0', 'vaex-ml': '0.14.0'}
Vaex was installed via: conda-forge + mamba !!!!!! :)
OS: Ubuntu 20.04

vaexio / vaex

[BUG-REPORT] join(): rprefix not systematically taken into account #1590