[Tex/LaTex] paragraph indent and line spacing

Question

In my text, I let make the paragraph indent automatically by LaTeX. That means everywhere I want to have a paragraph indentation in my text, I just press the enter key twice. However, when I look at the PDF version, I see that the line spacing above and below the paragraph input lines are different. Do you know how to fix this? I circled it red in the picture.

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{float}
\usepackage{caption}
\usepackage{booktabs}
\usepackage{graphicx}
\usepackage{threeparttable} 
\usepackage{caption} \captionsetup[table]{skip=4pt}
\usepackage{caption} \captionsetup[figure]{skip=3pt}
\usepackage{threeparttablex}
\usepackage[T1]{fontenc}
\usepackage[a4paper, left=2.5cm, right=3.5cm, top=2.5cm, bottom=2.5cm]{geometry}
\usepackage{setspace}
\usepackage{mwe}
\usepackage[toc,page]{appendix}
\onehalfspacing
\usetikzlibrary{intersections}
\usepackage{hanging}
\usepackage[bottom,hang]{footmisc}
\setlength{\footnotemargin}{10pt}
\usepackage{indentfirst}
\begin{document}

……

\end{document}

Answer 1

Your example code not valid MWE. If you change the dots by six medium paragraphs (or easier: \lipsum[1-6] with \usepackage{lipsum} in the preamble) you will not see that effect.

Adding a \parskip like 0pt minus 1 ex before of the paragraphs (in the preamble or after that) allow sometimes a negative paragraph skip (note that with only four paragraphs, i.e. \lipsum[1-4], that does not change anything) that look like your problem (see image bellow).

In you real document you probably have something like \setlength\parskip{0pt minus 1 ex} or something else (some other package, for example) changing that glue.

You can test the real values of \parskip at any point (for instance, after
the word "Spain", including \the\parskip. If this renders some like "0.0ptminus5.165pt" you should expect sometimes some lost of space between paragraphs.

In case of default values (0.0pt plus 1.0pt) or any other positive paragraph skip glue, then the problem is hard to guess, except if you provide a real MWE, reducing your real document as far as possible while still showing the problem but without left any irrelevant code (as probably, many of the listed packages). Probably some as simple like:

\documentclass[12pt]{article}
\usepackage[a4paper, left=2.5cm, right=3.5cm, top=2.5cm, bottom=2.5cm]{geometry}
\usepackage{setspace}
\onehalfspacing
\usepackage{hanging}
\usepackage{lipsum}
\setlength\parskip{0pt minus 1 ex}
\begin{document}
\lipsum[1-7]
This document have a park skip of \the\parskip
\end{document}